INT 16 - Shamrock Software EMAIL - CHECK FOR USER FUNCTION COMMAND-WORD         
	AX = 4504h
	DL = port number (01h = COM1)
	ES:BX -> 80-byte buffer for ASCIZA NUL-terminated ASCII string.	The ASCIZ string "ABC" consists of the four bytes 41h, 42h, 43h, and 00h.  Unless otherwise specified, maximum lengths given in the interrupt list do not include the terminating NUL. user input line
Return: AX = 4D00h if EMAIL installed on specified port
	    DL = flags
		bit 0: user function supported (always set)
		bit 1: user entered user-function command word
	    if DL bit 1 set,
		ES:BX buffer contains line entered by user which begins with
			the defined command word and has been converted to all
			caps
Note:	caller must process the returned commandline and invoke AX=4505h
	  within five seconds with the result of that processing
SeeAlso: AX=4503h,AX=4505h